The term “power factor” is borrowed from elementary AC circuit theory. When a sinusoidal AC power source feeds either an inductive or a capacitive load, the load current is also sinusoidal but lags or leads the input voltage by some angle “x” for an rms input voltage Vi and rms input current Ii, the “apparent power” taken from the lines is ViIi. However, the actual power delivered to the load is only ViIi cos(x) is only the component of input current which is in phase with the voltage across the load resistance (Ii cos(x)) which contributes to the load power. The component of input current normal to the voltage across the load resistor (Ii sin(x)) does not contribute to the actual load power. At one phase of the AC input wave form, it represents power drawn from the input source which is temporarily stored in the reactive component of the load device. In a later phase of the input voltage, this stored current or energy is returned to the input source. But this excessive current, which does not contribute to low power, wastes power in the winding resistance of the input power source and a resistance of the input power line. In AC power circuit jargon, the magnitude of cos(x) is referred to as the “power factor.” It is desirable to keep the power factor as close to unity as possible, i.e., to keep the input line current sinusoidal and in phase with the sinusoidal input line voltage. The means to achieve is referred to as “power factor correction” (PFC).
In the field of switching regulators, any circuit configuration which causes input line current to be non-sinusoidal or even sinusoidal but out of phase with the sinusoidal input voltage or to have harmonics of the line voltage results in a lowered power factor and consequent waste of the power.
To date, most power factor correction circuits utilize fully analog components to realize the correction algorithms and energy is switched to the load in such a manner as to maintain an in phase condition so as to maximize or “correct” the power factor and in such a manner as to minimize line harmonics.